Dual Role of H ₂ S and Seawater Ions in Promoting Efficient Electron Transfer on Indium-Loaded TiO ₂ for Photocatalytic H ₂ Production

Hydrogen (H2) is gaining recognition as a renewable energy carrier, and seawater splitting offers a sustainable way to produce it. However, the process is complex due to the unclear effects of salt ions on photocatalysts. Meanwhile, toxic hydrogen sulfide (H2S) poses health risks, emphasizing the need for effective utilization. This study investigates the performance of an indium-loaded TiO2 photocatalyst (In/TiO2) for seawater splitting using H2S as a sacrificial agent, aiming to enhance H2 production while utilizing this hazardous gas. While indium loading enhanced light absorption and suppressed charge recombination, the presence of NaCl improved electron transfer and resulted in a more favorable reduction potential. Thus, H2 yields of 3% In/TiO2 in 3.5 wt % NaCl solution produced 427.21 μmol·g–1 compared to only 66.59 μmol·g–1 produced from pure water after 1 h of irradiation. Interestingly, replacing methanol with H2S resulted in further increase in H2 yield, reaching 733.10 μmol·g–1 from 3.5 wt % NaCl solution after 1 h of irradiation. At 0.5 wt % NaCl, the photocatalytic system achieved its highest hydrogen yield, whereas at 5 wt % the yield dropped sharply, falling to only about 54% of the value recorded at 0.5 wt %. Results also showed that individual seawater salts including NaCl, MgCl2, CaCl2, Na2SO4, and KCl, at concentrations similar to those in natural seawater each contributed to enhanced H2 production, though to varying degrees. Among them, MgCl2 exhibited the highest yield of 897.80 μmol·g–1 after 1 h of irradiation. The proposed mechanism revealed the dual role of seawater ions and H2S in enhancing H2 evolution over In/TiO2. Bisulfide ions scavenge photogenerated holes, while dissolved salts improve conductivity and charge transfer. Together, these effects enable a cost-effective and sustainable route for hydrogen production.